1. Field of the Invention
The present invention relates to a current detection unit of an inverter which outputs a pseudo three-phase AC power, converted from a DC power by pulse width modulation control (hereinafter “PWM control”) by a chopping wave comparison method. The inverter outputs the AC power to a load, such as a motor or a transformer. More specifically, the invention relates to a current detection unit which detects a three-phase current output from the inverter to the load by measuring a DC current flowing in the inverter.
2. Description of Related Art
FIG. 1 depicts an example of a known motor control unit using an inverter. In FIG. 1, a three-phase current is applied from an inverter 2 to a three-phase, brushless motor 1. A DC current is applied to inverter 2 from a DC power source 3. Inverter 2 is driven by a drive unit 4, and drive unit 4 is controlled by a control unit 5. The DC current applied to inverter 2 is detected by a DC current sensor 6, and the detected signal is sent to control unit 5 through an A/D converter 7.
Inverter 2 has three pairs of switching elements Us, Xs, Vs, Ys, Ws, and Zs, each comprising a transistor and the like. Us, Vs, and Ws are upper switching elements; and Xs, Ys, and Zs are lower switching elements. Through on/off control of the respective switching elements; in response to the PWM signals from drive unit 4, inverter 2 converts the DC power sent from DC power source 3 into a pseudo three-phase AC power and outputs the AC power to respective coil phases Uc, Vc, and Wc of motor 1. DC current sensor 6 is provided for measuring a DC current flowing in the power source wire of inverter 2 (hereinafter “power source current Idc”), and the measurement signal thereof is input to control unit 5 after that signal is A/D converted at A/D converter 7.
Three pairs of switching elements of inverter 2 are on/off controlled in response to PWM signals from control unit 4, and the on/off conditions may be classified into eight (8) configurations depicted in FIGS. 2A to 2H. In FIGS. 2A to 2H, each switching element is depicted as a simple switch for better understanding of the on/off condition control. Further, in FIGS. 2A to 2H, Idc indicates a power source current, Iu indicates a U-phase current output to U-phase coil Uc of motor 1, Iv indicates a V-phase current output to V-phase coil Vc of motor 1, and Iw indicates a W-phase current outputted to W-phase coil Wc of motor 1, respectively.
In the configuration depicted in FIG. 2A, because Xs, Ys, and Zs are ON and Us, Vs, and Ws are OFF, power source current Idc becomes zero. In the configuration depicted in FIG. 2B, because Us, Ys, and Zs are ON and because and Xs, Vs, and Ws are OFF, power source current Idc becomes Idc=Iu (=−Iv−Iw). In the configuration depicted in FIG. 2C, because Xs, Vs, and Zs are ON and because Us, Ys, and Ws are OFF, power source current Idc becomes Idc=Iv (=−Iu−Iw). In the configuration depicted in FIG. 2D, because Us, Vs, and Zs are ON and because Xs, Ys, and Ws are OFF, power source current Idc becomes Idc=Iu+Iv (=−Iw). In the configuration depicted in FIG. 2E, because Xs, Ys, and Ws are ON and Us, Vs, and Zs are OFF, power source current Idc becomes Idc=Iw (=−Iu−Iv). In the condition depicted in FIG. 2F, because Us, Ys, and Ws are ON and because Xs, Vs, and Zs are OFF, power source current Idc becomes Idc=Iw+Iu (=−Iv). In the configuration depicted in FIG. 2G, because Xs, Vs, and Ws are ON and because Us, Ys, and Zs are OFF, power source current Idc becomes Idc=Iv+Iw (=−Iu). In the configuration depicted in FIG. 2H, because Us, Vs, and Ws are ON and because Xs, Ys, and Zs are OFF, power source current Idc becomes zero.
Specifically, by measuring the power source current Idc in the respective conditions, except the conditions depicted in FIGS. 2A and 2H, Iu in the conditions depicted in FIGS. 2B and 2G, Iv in the conditions depicted in FIGS. 2C and 2F, and Iw in the conditions depicted in FIGS. 2D and 2E, are obtained as respective phase currents, respectively. Therefore, three phase currents Iu, Iv, and Iw may be determined by measuring power source current Idc under either of two variations on each of three configurations of FIG. 2B or 2G, FIG. 2C or 2F, and FIG. 2D or 2E.
The timing chart depicted in FIG. 3 shows a known current detection method based on the above-described concept (for example, Japanese Patent No. 2,563,226 or JP-A-10-155278). In FIG. 3, BTW indicates a reference chopping wave with a predetermined frequency, SVw indicates a comparison reference signal for setting a W-phase output, SVv indicates a comparison reference signal for setting a V-phase output, and SVu indicates a comparison reference signal for setting a U-phase output. Similarly, OSu indicates a U-phase output set by reference chopping wave BTW and comparison reference signal SVu, OSv indicates a V-phase output set by reference chopping wave BTW and comparison reference signal SVv, and OSw indicates a W-phase output set by reference chopping wave BTW and comparison reference signal SVw.
In this current detection method, power source current Idc is measured at a time corresponding to one switching condition among the above-described conditions depicted in FIGS. 2B to 2G, and one phase current is detected. The power source current Idc is measured at a time corresponding to a switching condition different from the above-described switching condition, and another phase current different from the above-described phase current is detected. The remaining, one phase current is calculated from the detected, two phase currents, thereby accomplishing the desired current detection.
In the above-described, known current detection method, however, because power source current Idc is measured basically at a time at which the switching condition is switched, as shown in FIG. 3, when two measurement times t1 and t2 are close to each other, it may be difficult to measure the power source current Idc at the first measurement time t1.
Specifically, as shown in FIG. 4, in a case in which a signal Is for switching the on/off condition of an arbitrary pair of switching elements is sent from drive unit 4 to inverter 2, an upper switching element USE is switched from ON to OFF at a time later than a time at which the signal is switched from a high level to a low level, a lower switching element LSE is switched from a low level to a high level at a time later than the above-described time, and as a result of this switching operation, the power source current Idc varies. Further, because the measurement signal of DC current sensor 6, which comprises a resistance and the like, is output to control unit 5 after being A/D converted by A/D converter 7, a time delay occurs in obtaining the A/D converted output (shown as “A/D” in FIG. 4) and accompanies the variation of the power source current Idc. Because the time td shown in FIG. 4 becomes about on and one half micro (1.5μ) seconds in the situation in which a general insulated, gate bipolar transistor (IGBT) element is used as a switching element, a time interval of at least about td is required between measurement times t1 and t2 in order to accurately measure the power source current Idc at measurement time t1. Therefore, when the time difference between measurement times t1 and t2 are less than the time td shown in FIG. 4, even if the power source current Idc is measured at measurement time t1, the measured value may include an error.
Further, in the above-described, known, current detection method, because the power source current Idc is measured at a time at which the switching condition is switched, if the duty ratios of the respective phase outputs vary, the measurement interval INT shown in FIG. 3 increases or decreases, and, therefore, it may be difficult to obtain phase current information over a consistent measurement period.